Magnetic head having a slider made of a glass-ceramic

ABSTRACT

A magnetic head has a slider made of a glass-ceramic substrate which has, as its predominant crystal phase, a mixed crystal of lithium disilicate (Li 2 O.2SiO 2 ) and either or both of α-quartz (α-SiO 2 ) and α-cristobalite (α-SiO 2 ), the α-quartz has a globular grain structure each globular crystal grain being made of aggregated particles and having a grain diameter within a range from 0.1 μm to 3.0 μm, the α-cristobalite has a globular grain structure each globular grain having a grain diamter within a range from 0.1 μm to 1.0 μm, and surface roughness (Ra) of the glass-ceramic substrate after polishing is within a range from 5 Å to 50 Å.

RELATED PATENT APPLICATION

This application is a continuation-in-part of our prior application U.S.Ser. No. 08/924,344, filed on Sep. 5, 1997, which claims priority toJapanese Applications Nos. 255447/1996 filed Sep. 9, 1996; 327689/1996,filed Nov. 22, 1996 and 251389/1997 filed Sep. 1, 1997. This applicationis also related to Ser. No. 09/266,559 filed Mar. 11, 1999 now U.S. Pat.No. 6,034,011 which is a division of Ser. No, 08/924,344, filed Sep. 5,1997, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates a magnetic head having excellent machineprocessability and surface characteristics and having a good frictioncharacteristic with respect to a magnetic disk.

Recent development of multimedia technology has resulted in processingof a large volume of data for image and voice. To cope with thissituation, a rapid progress has been made for changing an HDD (harddisk) device which is a main external recording medium of a computer toone of a larger capacity and a higher speed. Keeping pace with thistendency, an inductive type thin film magnetic head is being replaced bya magneto-resistive head (MR head) in the field of a thin film magnetichead. Further, as a future type of magnetic head, a giantmagneto-resistive head (GMR head) has become an object of attention.

In operation, the current hard disk device rotates a hard disk at a highspeed and performs writing on and reading from the hard disk by means ofa magnetic head which in operation is not in contact with the surface ofthe hard disk. In a hard disk device employing a CSS (contact start andstop) system, in order to prevent sticking of the magnetic head sliderto the magnetic disk, a processing (mechanical texturing) is performedaccording to which a mechanical grinding is made for leavingsubstantially concentric circumferential machining traces on a magneticdisk substrate. There are also performed alternative texturingprocessings on a magnetic disk such as an etching processing, asputtering processing and a laser processing. It is also proposed toproduce crystal grains having a special crystal structure on the surfaceof a disk substrate and thereby obviate the above mentioned texturingprocessings. As to a magnetic head, the following proposals have beenmade for coping with the problem relating to the CSS system.

Japanese Patent Application Laid-open No. Sho 56-169264 proposes amagnetic head in which a non-contact type film is formed on a slidersurface which opposes the surface of a magnetic medium for preventingsticking of the slider. Japanese Patent Application Laid-open No. Hei5-182189 proposes a method for preventing sticking by forming aphoto-sensitive glass layer on a rail port ion of a magnetic head whichcontacts the surface of a magnetic disk substrate and forming a patternof projections and depressions on the surface of this photo-sensitiveglass layer. Japanese Patent Application Laid-open No. Sho 63-60127discloses high strength and high thermal expansion glass-ceramics for amagnetic head substrate having α-quartz and spinel as its predominantcrystal phase.

However, in the magnetic head having a non-contact type film, powderproduced as a result of wear of the film is liable to cause difficultyin the sliding movement in case the film is made of a solid materialwhile, in case the film is made of a liquid material, it is liable tocause wear and stripping off due to wear and resulting sticking to theslider with resulting decrease in efficiency.

In the magnetic head using the photo-sensitive glass, there is a problemof stripping off of the glass layer from the magnetic head. Besides, thephoto-sensitive processing or etching processing must be made forforming the pattern of projections and depressions which islabor-consuming and costly. The glass-ceramics having α-quartz andspinel as its crystal phase has a high strength but its processabilityis poor. There is also a magnetic head material which uses analumina-titanium-carbide sintered material but this material is veryhard and its processability is poor. Moreover, this material has manyvoids and this reduces productivity and make it difficult to manufacturethe magnetic head with this material at a low cost.

It is an object of the present invention to eliminate the abovedescribed defects of the prior art and provide a magnetic head havingexcellent surface characteristics.

SUMMARY OF THE INVENTION

Accumulated studies and experiments made by the inventors of the presentinvention for achieving the above described object of the invention haveresulted in the finding, which has led to the present invention, that aglass-ceramic obtained by subjecting to heat treatment, under specificheating conditions, a base glass of SiO 2 —Li 2 O—P 2 O 5 system has amixed crystal structure in which either or both of globular crystalgrains of α-quartz (α-SiO 2 ) each consisting of aggregated particlesand globular grains of a -cristobalite have randomly grown in a finecrystal of lithium disilicate (Li2O.2SiO 2 ) and that differences inmechanical and chemical actions caused by the polishing processingbetween the lithium disilicate crystal phase which is mechanically andchemically instable and the aggregated globular grains of α-quartz orglobular grains of α-cristobalite which is mechanically and chemicallystable cause projections and depressions to be produced on the surface,and further that, by controlling the size of either or both of theaggregated globular grains of α-quartz and the globular grains ofα-cristobalite, a material having excellent surface characteristicsafter polishing as the magnetic head substrate can be obtained.

For achieving the above described object of the invention, there isprovided a magnetic head having a slider comprising a glass-ceramicsubstrate which has as its predominant crystal phase a mixed crystal oflithium disilicate (Li 2 O.2SiO 2 ) and either or both of α-quartz(α-SiO 2 ) and α-cristobalite (α-SiO 2 ), said α-quartz having aglobular grain structure each globular crystal grain being made ofaggregated particles and having a grain diameter within a range from 0.1μm to 3.0 μm, said α-cristobalite having a globular grain structure eachglobular grain having a grain diamter within a range from 0.1 μm to 1.0uμm, and surface roughness (Ra) of said glass-ceramic substrate afterpolishing being within a range from 5 Å to 50 Å.

In one aspect of the invention, the glass-ceramic substrate of themagnetic head is obtained by subjecting to heat treatment a base glasscomprising in weight percent:

SiO₂ 65-83% Li₂O  8-13% K₂O 0-7% MgO + ZnO + PbO 0.5-5.5% in which MgO0.5-5.5% ZnO 0-5% PbO 0-5% P₂O₅ 1-4% Al₂O₃ 0-7% As₂O₃ + Sb₂O₃ 0-2%

and thereafter polishing the surface of the substrate to surfaceroughness (Ra) of 5 Å to 50 Å.

In another aspect of the invention, the glass-ceramic substrate for amagnetic head is characterized in that said glass-ceramic substrate isobtained by subjecting to heat treatment a base glass comprising inweight percent:

SiO₂ 70-82% Li₂O  8-12% K₂O 1-6% MgO + ZnO 1.5-5.5% in which MgO 1-5%ZnO 0.2-5%   P₂O₅ 1-3% Al₂O₃ 1-6% As₂O₃ + Sb₂O₃  0-2%.

and thereafter polishing the surface of the substrate to surfaceroughness (Ra) of 5 Å to 50 Å.

According to the invention, the glass-ceramic substrate of the magnetichead has as its predominant crystal phase a mixed crystal of lithiumdisilicate (Li 2 O.2SiO 2 ) and either or both of α-quartz (α-SiO 2 )and α-cristobalite (α-SiO 2 ), the α-quartz has a globular grainstructure each globular crystal grain being made of aggregated particlesand having a grain diameter within a range from 0.1 μm to 3.0 μm, theα-cristobalite has a globular grain structure each globular grain havinga grain diamter within a range from 0.1 μm to 1.0 μm, and surfaceroughness (Ra) of the glass-ceramic substrate after polishing is withina range from 5 Å to 50 Å whereby the invention can provide a magnetichead having a slider which has a surface rougness which does not needthe laser texturing or mechanical or chemical textureing which has beenrequired in the prior art magnetic head substrates for improving the CSScharacteristics of the magnetic disk device.

Further, since the surface roughness within the above described rangecan be easily obtained by simply changing the conditions of heattreatment, the substrate can be adapted for use with magnetic disksubstrates having a variety of surface roughnesses. Furthermore, thesubstrate surface has a fine crystal structure having no surface defectsuch as a pit or void. Therefore, a magnetic head having characteristicssuited for the high recording density magnetic disk device can bemanufactured at a low cost and in a large scale on a stable basis.

In the magnetic head according to the invention, it is essential fromthe standpoint of tribology between the magnetic head and the magneticdisk that the glass-ceramic substrate has as its predominant crystalphase a mixed crystal of lithium disilicate (Li 2 O.2SiO 2 ) and eitheror both of α-quartz (α-SiO 2 ) and α-cristobalite (α-SiO 2 ), saidα-quartz has a globular grain structure each globular crystal grainbeing made of aggregated particles and having a grain diameter within arange from 0.1 μm to 3.0 μm, said α-cristobalite has a globular grainstructure each globular grain having a grain diameter within a rangefrom 0.1 μm to 1.0 μm, and surface roughness (Ra) of said glass-ceramicsubstrate after polishing is within a range from 5 Å to 50 Å.

Preferably, the size of the aggregated globular grain of α-quartz shouldbe 0.1 μm-2.0 μm, the size of the globular grain of α-cristobaliteshould be 0.1 μm-0.8 μm and the surface roughness after polishing shouldbe 5 Å-40 Å. More preferably, the size of the aggregated globular grainof α-quartz should be 0.1 μm-1.5 μm, the size of the globular grain ofα-cristobalite should be 0.1 μm-0.6 μm and the surface roughness afterpolishing should be 5 -30 Å.

BRIEF DESCRIPTION OF THE DRAWING

In the accompanying drawing, FIG. 1 is a perspective view of a magnetichead embodying the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an embodiment of a magnetic head of the present invention.In FIG. 1, a magnetic head 1 includes a slider 2 and a magnetic core 3.The slider 2 has a pair of parallel rails 4, 4 and can be mounted on amagnetic head suspension unit (not shown) by having the rails 4, 4engaged with a corresponding guide portion provided in the magnetic headsuspension unit. The magnetic core 3 is made of magnetic substance suchas ferrite and is fixed to the slider 2. The slider 2 is made of aglass-ceramic substrate.

The composition of the glass-ceramic substrate which constitutes theslider 2 of the magnetic head 1 of the present invention is expressed onthe basis of composition of oxides as in its base glass. The abovedescribed content ranges of the respective ingredients have beenselected for the reasons stated below.

The SiO 2 ingredient is a very important ingredient which produceslithium di silicate (Li 2 O.2SiO 2 ), α-quartz (α-SiO 2 ) andα-cristobalite crystal as a predominant crystal phase when the baseglass is subjected to heat treatment. if the amount of this ingredientis below 65%. the crystal produced in the glass-ceramic is instable andits texture tends to become too rough. If the amount of this ingredientexceeds 83%, difficulty arises in melting and forming the base glass. Asa result of experiments, it has been found that a preferable range is70-82% and a more preferable range is 70-77%.

The Li 2 O ingredient is also an important ingredient which produceslithium disilicate (Li 2 O.2SiO 2 ) crystal as a predominant crystalphase by heat treating the base glass. If the amount of this ingredientis below 8%, difficulty arises in growing of this crystal and also inmelting of the base glass. If the amount of this ingredient exceeds 13%,texture of the grown crystal of the glass-ceramic tends to become toorough and besides, chemical durability and hardness of thisglass-ceramic are deteriorated. A preferable range is 8-12% and a morepreferable range is 8-11%.

The K 2 O ingredient improves melting property of the base glass andprevents crystal grains from becoming too rough. If the amount of thisingredient exceeds 7%, the grown crystal becomes too rough, the crystalphase undergoes change and chemical durability is deteriorated so thatthe amount should be 7% or below. A preferable ragne is 1-6% and a morepreferable range is 1-5%.

The divalent metal oxides MgO, ZnO and PbO are important ingredientswhich cause crystal grains of α-quartz (α-SiO 2 ) to grow randomly as apredominant crystal phase in the glass-ceramic of the invention in theform of globular grains each consisting of aggregated particles. As aresult of experiments, it has been found that, in order to cause adesired crystal to grow, it is necessary to add the MgO ingredient in arange from 0.5-5.5% and the ZnO and PbO ingredients may be added up to5% respectively for assisting adjustment of growth of the crystal. Ifthe total amount of these three ingredients is below 0.5%, a desiredgrain diameter cannot be obtained whereas if the total amount of theseingredients exceeds 5.5%, difficulty arises in obtaining a desiredcrystal. A preferable range is 1-5% for MgO, 0.2-5% for ZnO and 1.5-5.5%for MgO+ZnO. A more preferable range is 2-4% for MgO, 0.5-4% for ZnO and2.5-5.5% for MgO+ZnO.

The P 2 O 5 ingredient is an essential ingredient as a nucleating agentfor the glass-ceramic. If the amount of this ingredient is below 1%,sufficient forming of nucleus cannot be attained. If the amount of thisingredient exceeds 4%, the crystal grown of the glass-ceramic becomesinstable and its texture becomes too rough and, moreover,devitrification tends to occur during melting of the base glass. Apreferable range is 1-3% and a more preferable range is 1.5-2.5%.

The Al 2 O 3 ingredient is effective for improving chemical durabilityof the glass-ceramic. If the amount of this ingredient exceeds 7%, themelting property of the glass is deteriorated and the amount ofprecipitation of α-quartz as a predominant crystal phase is reduced. Apreferable range is 1-6%.

The AS2 O 3 and Sb 2 O 3 ingredients may be added as refining agents inmelting of the base glass. It will suffice if each ingredient up to 2%is added.

In addition to the above described ingredients, a small amount of B 2 O3 , CaO, SrO, BaO, TiO 2 , SnO 2 and ZrO 2 may be added for adjustingthe diameter of grown crystal grains within a range not impairing thedesired characteristics of the glass-ceramics.

For manufacturing the glass-ceramic substrate for a magnetic head of theinvention from the base glass having the above described composition,the base glass having the above described composition is melted and issubjected to heat forming or cold forming and, thereafter, is heattreated for forming a crystal nucleus at a temperature within a rangefrom 400° C. to 600° C. and further heat treated for crystallization ata temperature within a range from 650° C. to 850° C. and theglass-ceramic substrate can be easily obtained. If the nucleatingtemperature is below 400° C., forming of nucleus by phase-separation ofP 2 O 5 ingredient becomes insufficient whereas if the nucleatingtemperature exceeds 600° C., a uniform forming of crystal nucleus offine crystal of lithium disilicate cannot be achieved and, moreover, thecrystal nucleus becomes too rough. As a result, the aggregated globulargrains of α-quartz (α-SiO 2 ) which precipitate subsequently tend tobecome dispersed and become single crystal grains of α-quartz andcrystal grains of α-cristobalite having an abnormal diameter tend to beproduced.

The crystallization temperature is as important as the effect ofaddition of the divalent metal oxides including MgO ingredient forcontrolling the α-quartz crystal to the globular grains of aggregatedparticles and controlling the α-cristobalite crystal to the globulargrains. If the crystallization temperature is below 650° C., difficultyarises in aggregating of fine particles of the α-quartz crystal into theglobular grain structure. If the crystallization temperature exceeds850° C., it becomes difficult to maintain the globular grain structureof aggregated particles of α-quartz and also the globular grainstructure of α-cristobalite. For obtaining the best surface roughness asthe magnetic head substrate, the base glass should preferably be heattreated at the nucleating temperature within a range from 450° C. to540° C. for one to five hours and further heat treated at thecrystallization temperature within a range from 730° C. to 820° C. forone to five hours.

The glass-ceramic obtained by the above described heat treatment issubjected to conventional lapping and polishing processes and thereby aglass-ceramic substrate for a magnetic head having a surface roughness(Ra) within a range from 5 Å to 50 Å is provided.

EXAMPLES

Tables 1-6 show results of measurements of crystal phase and surfaceroughness (Ra) after polishing of glass-ceramics of the invention whichwere manufactured from base glasses having compositions of Examples Nos.1-22 and Comparative Examples 1 and 2 under the conditions described inthe tables (i.e., nucleation temperature, crystallization temperatureand processing hour). In the tables, numerical values of respectiveingredients are expressed in weight percent, α-quartz is indicated as“α-Q” and α-cristobalite as “α-C” respectively.

TABLE 1 Example No. 1 2 3 4 SiO₂ 69.0 73.0 76.7 75. 5 Li₂O 9.0 8.0 10.010.0 Al₂O₃ 5.0 5.0 3.8 4.0 MgO 3.5 4.0 3.0 1.5 ZnO 0.5 0.5 0.5 PbO 0.52.0 K₂O 7.0 4.0 3.5 4.5 P₂O₅ 1.5 4.0 2.0 2.0 As₂O₃ 0.5 0.5 0.5 0.5 Sb₂O₃others BaO:1.5 SrO:1.0 ZrO₂:1.0 nucleation 450 540 480 500 temperature(° C.) nucleation 2 5 5 1 time (hr) crystallization 760 780 750 750temperature (° C.) crystallization 3 3 8 2 time (hr) predominant α-Q α-Qα-Q α-Q crystal phase (globular) (globular) (globular) (globular) graindiameter 0.6 1.3 1.2 0.7 (μm) Li₂O.2SiO₂ Li₂O.2SiO₂ Li₂O.2SiO₂Li₂O.2SiO₂ surface 15 35 32 18 roughness Ra (Å)

TABLE 2 Example No. 5 6 7 8 SiO₂ 76.0 76.5 77.5 82.0 Li₂O 10.5 11.0 9.011.0 Al₂O₃ 4.0 3.0 2.0 1.5 MgO 2.5 1.0 2.5 1.0 ZnO 0.5 2.0 1.0 PbO 1.0K₂O 4.0 4.0 5.0 2.3 P₂O₅ 2.0 2.0 1.5 1.7 AS₂O₃ 0.5 0.5 0.5 0.5 Sb₂O₃others nucleation 500 540 500 540 temperature (° C.) nucleation 5 5 5 5time (hr) crystallization 780 780 730 820 temperature (° C.)crystallization 2 5 2 5 time (hr) predominant α-Q α-Q α-Q α-Q crystalphase (globular) (globular) (globular) (globular) grain diameter 1.2 1.50.5 1.3 (μm) Li₂O.2SiO₂ Li₂O.2SiO₂ Li₂O.2SiO₂ Li₂O.2SiO₂ surface 30 4012 33 roughness Ra (Å)

TABLE 3 Example No. 9 10 11 12 SiO₂ 76.7 75.0 67.5 69.0 Li₂O 10.5 11.010.0 9.0 Al₂O₃ 3.5 3.8 6.0 5.0 MgO 2.8 3.0 2.5 3.5 ZnO 0.5 1.0 1.0 0.5PbO 1.5 1.5 K₂O 4.0 4.5 6.0 7.0 P₂O₅ 2.0 2.0 2.0 1.5 AS₂O₃ 0.5 0.5 0.5Sb₂O₃ 0.5 others CaO:1.0 BaO:1.5 B₂O₃:2.0 ZrO₂:1.0 nucleation 540 540480 510 temperature (° C.) nucleation 5 5 3 1 time (hr) crystallization780 760 750 760 temperature (° C.) crystallization 2 2 2 3 time (hr)predominant α-Q α-Q α-Q α-Q crystal phase (globular) (globular)(globular) (globular) grain diameter 0.8 0.9 0.7 0.6 (μm) Li₂O.2SiO₂Li₂O.2SiO₂ Li₂O.2SiO₂ Li₂O.2SiO₂ surface 20 22 16 15 roughness Ra (Å)

TABLE 4 Example No. 13 14 15 16 SiO₂ 69.5 73.0 74.0 75.5 Li₂O 9.0 8.012.5 10.0 Al₂O₃ 5.5 5.0 4.5 4.5 MgO 1.0 4.0 2.0 1.0 ZnO 0.5 0.5 PbO 4.02.0 2.0 K₂O 6.0 4.0 4.5 P₂O₅ 2.0 4.0 2.5 2.0 AS₂O₃ 0.5 0.5 0.5 Sb₂O₃ 0.5others TiO₂:2.0 SrO:1.0 SnO₂:2.0 nucleation 470 530 500 430 temperature(° C.) nucleation 6 3 4 8 time (hr) crystallization 770 780 780 750temperature (° C.) crystallization 2 3 3 2 time (hr) predominant α-Q α-Qα-Q α-Q crystal phase (globular) (globular) (globular) (globular) graindiameter 0.7 1.3 1.5 0.8 (μm) Li₂O.2SiO₂ Li₂O.2SiO₂ Li₂O.2SiO₂Li₂O.2SiO₂ surface 18 35 42 18 roughness Ra (Å)

TABLE 5 Example No. 17 18 19 20 SiO₂ 76.0 76.5 78.5 82.3 Li₂O 10.5 10.59.0 12.0 Al₂O₃ 4.0 3.5 2.0 MgO 2.5 1.0 2.5 1.0 ZnO 0.5 2.0 1.0 PbO 1.0K₂O 4.0 4.0 4.0 2.5 P₂O₅ 2.0 2.0 1.5 1.7 AS₂O₃ 0.5 0.5 0.5 0.5 Sb₂O₃others nucleation 470 580 520 450 temperature (° C.) nucleation 4 2 4 3time (hr) crystallization 780 780 780 820 temperature (° C.)crystallization 2 5 2 2 time (hr) predominant α-Q α-Q α-Q α-Q crystalphase (globular) (globular) (globular) (globular) grain diameter 1.2 1.51.4 1.0 (μm) Li₂O.2SiO₂ Li₂O.2SiO₂ Li₂O.2SiO₂ Li₂O.2SiO₂ surface 30 4036 20 roughness Ra (Å)

TABLE 6 Comparative Comparative Example No. 21 22 Example 1 Example 2SiO₂ 76.5 76.5 56.0 82.3 Li₂O 10.5 10.0 12.0 Al₂O₃ 3.5 3.5 22.5 9.6 MgO0.8 1.3 10.5 ZnO 0.5 0.5 PbO 0.9 K₂O 3.8 3.8 P₂O₅ 2.3 2.3 1.9 AS₂O₃ 0.5Sb₂O₃ 0.2 0.2 0.2 others ZrO₂:1.9 ZrO₂:1.9 TiO₂:6.5 Na₂O:3.8 ZrO₂:2.5CaO:0.4 BaO:1.5 SrO:0.4 nucleation 540 530 820 600 temperature (° C.)nucleation 3 2 0.5 3 time (hr) crystallization 760 740 1020 840temperature (° C.) crystallization 2 1 8 2 time (hr) predominant α-C α-Qα-Q α-Q crystal phase (globular) (globular) (globular) (globular) graindiameter 0.3 0.3 spinel 3.5 (μm) Li₂O.2SiO₂ α-C Li₂O.2SiO₂ (globular)0.1 Li₂O.2SiO₂ surface 5 5 53 65 roughness Ra (Å)

For manufacturing the glass-ceramic subs t rate for magnetic head of theabove described examples, materials including oxides, carbonates andnitrates are mixed and molten in a conventional melting apparatus at atemperature within a range from about 1350° C. to about 1500° C. Themolten glass is stirred to homogenize it and thereafter formed to adesired shape and annealed to provide a formed glass. Then, this formedglass is subjected to heat treatment (nucleating and crystallizingprocesses) at a predetermined temperature and for a predetermined hourto provide a desired glass-ceramic. The glass-ceramic is then subjectedto lapping with GC lapping grains and finally polished for about 30minutes to 40 minutes with cerium oxide having average grain diameterranging from 9 μm to 12 μm. The surface roughness (Ra) was measured witha surface roughness meter (Tencor P2) and identification of the growncrystal was made with an X-ray diffraction apparatus.

As shown in the Examples, in the glass-ceramic for a slider of amagnetic head, a desired surface roughness can be obtained by causingeither or both of globular grains of α-quartz each consisting ofaggregated particles and globular grains of α-cristobalite to randomlygrow in the fine crystal of lithium disilicate (Li 2 O.2SiO 2 ). On theother hand, in the glass-ceramic substrate of Comparative Example 1 inwhich the grown crystal phases are α-quartz and spinel, a desiredsurface roughness cannot be achieved. Moreover, since thecrystallization temperature and the nucleating temperature are muchhigher than those of the present invention, the glass-ceramic ofComparative Example 1 has only a poor productivity. In the glass-ceramicsubstrate of Comparative Example 2 in which the grown crystal phases areα-quartz and lithium disilicate, the grain diameter of the grown crystalis too large to provide a desired surface roughness.

What is claimed is:
 1. A magnetic head having a slider comprising aglass-ceramic substrate which has as its predominant crystal phase amixed crystal of lithium disilicate (Li 2 O.2SiO 2 ) and either or bothof α-quartz (α-SiO 2 ) and α-cristobalite (α-SiO 2 ), said α-quartzhaving a globular grain structure each globular crystal grain being madeof aggregated particles and having a grain diameter within a range from0.1 μm to 3.0 μm, said α-cristobalite having a globular grain structureeach globular grain having a grain diameter within a range from 0.1 μmto 1.0 μm, and surface roughness (Ra) of said glass-ceramic substrateafter polishing being within a range from 5 Å to 50 Å.
 2. A magnetichead as defined in claim 1 wherein said glass-ceramic substrate isobtained by subjecting to heat treatment a base glass comprising inweight percent: SiO₂ 65-83% Li₂O  8-13% K₂O 0-7% MgO + ZnO + PbO0.5-5.5% in which MgO 0.5-5.5% ZnO 0-5% PbO 0-5% P₂O₅ 1-4% Al₂O₃ 0-7%As₂O₃ + Sb₂O₃  0-2%.


3. A magnetic head as defined in claim 1 wherein said glass-ceramicsubstrate is obtained by subjecting to heat treatment a base glasscomprising in weight percent: SiO₂ 70-82% Li₂O  8-12% K₂O 1-6% MgO + ZnO1.5-5.5% in which MgO 1-5% ZnO 0.2-5%   P₂O₅ 1-3% Al₂O₃ 1-6% As₂ + Sb₂O₃ 0-2%.